A conventional package material for a snack food frequently has a 5-layer structure consisting of polypropylene resin layer/polyethylene resin layer/metal aluminum vapor deposition—biaxial oriented PET (polyethylene terephthalate) layer/polyethylene layer/biaxial oriented polypropylene resin layer in this order from the inside.
A polypropylene resin (PP) layer as the innermost layer serves as a sealant layer. For such an innermost layer, a polypropylene resin is selected since a low temperature heat seal ability and a hot tack property immediately after the heat sealing. The hot tack property is required because of a slightly pressurized packaging process conducted while inflating a bag with air or nitrogen for the purpose of preventing any damage on a snack food as a content in the bag. The thickness of the polypropylene resin layer as this innermost layer is frequently about 20 to 50 μm.
A metal aluminum vapor deposition layer serves to shield an external light and to prevent the permeation of oxygen and water vapor. It has a thickness of 300 to 1500 angstrom and is formed usually by a vapor deposition of a metal aluminum onto a biaxial oriented PET film or a polypropylene resin.
A biaxial oriented PET layer consists of a biaxial oriented polyethylene terephthalate (PET) film, which is a material onto which a metal aluminum can be deposited most stably and frequently has a thickness of about 12 μm.
A polyethylene resin (PE) layer serves as an adhesive for binding a biaxial oriented PET film with an outermost biaxial oriented polypropylene (OPP) film or for binding an innermost non-oriented polypropylene resin with a biaxial oriented PET film, and has a thickness of about 15 μm. The polyethylene resin layer sometimes employs an ethylene-(meth)acrylic acid copolymer, and the polyethylene resin layer is replaced sometimes with an urethane-based adhesive or an organic solvent-free monomer-containing adhesive.
A biaxial oriented polypropylene film as an outermost layer is brought into a direct contact with a heat sealing device (seal bar) at an elevated temperature, and serves to transmit the heat to the innermost polypropylene resin layer. It has a thickness of 15 to 25 μm.
A package material for a snack food having 4 to 5 layers as described above has a total thickness of 40 to 117 μm.
A method for fabricating a packaging material having the above mentioned layer construction may frequently, when taking a cost into consideration, involve a fabrication procedure in which a PET film is subjected to a vapor deposition with a metal aluminum, and then a PE (polyethylene) is extruded between the PET film vapor deposition side and the non-oriented polypropylene film to effect a lamination while extruding a PE between an OPP film and the PET film (the opposite side of the vapor deposition side) whereby effecting a lamination.
However, such a package structure poses an extreme difficulty in recycling the materials because of the combination of a diversity of materials such as polypropylene resins, PET resins, polyethylene resins and the like. In addition, a polypropylene resin forming an innermost layer adsorbs the flavors of a snack food, resulting in a deteriorated taste. A further disadvantage is that a total thickness as thick as about 40 to 120 μm makes it difficult to transmit a heat.
Recently, a demand of increasing the producibility leads to a demand of a higher speed of a bag making and packaging machine. In order to accomplish a high speed production, a heat seal should be accomplished within a shorter seal time, resulting naturally in a demand of a heat seal process at a high temperature within a short time for giving a certain calorie. Thus, a high seal temperature is required to give, within a shorter seal time, a calorie given over conventional seal time, but conventional bag material layer construction can not be sealed at a high temperature because of a small difference in the melting point between the outermost and the innermost layers.
On the other hand, such a plastic packaging material, which has been employed for a long time, now poses an waste disposal problem after being used. Generally, a plastic waste is collected as a refuse, which is then incinerated or dumped in the ground.
When incinerated, a conventional plastic packaging material consisting mainly of polyolefins gives a burning calorie as high as 4.2×107 J/kg or more, which leads to a high possibility of damaging a furnace, sometimes in combination with a possibility of emitting dioxins depending on the incineration temperature when containing chlorinated compounds, resulting in a controversy about the incineration itself.
When dumped in the ground, a polyolefin such as a polypropylene or polyethylene remains in the nature semi-permanently, resulting in a cause of an environmental pollution.